For many applications of high-T.sub.c superconductors, such as field-effect devices, high-quality thin films of high-T.sub.c superconductor materials are required. The quality of these superconductor films depends to a large extent on the substrate onto which the superconductor material is deposited. In particular, the quality of the high-T.sub.c superconductor films depends on the degree of match between the lattice parameters of the substrate material and the lattice parameters of the superconductor film material. For film thicknesses below about 100 nm, the importance of this match increases with decreasing thickness of the superconductor films.
For the purposes of the description to follow, the term substrate shall be deemed to encompass the bulk material, relatively thick for reasons of mechanical stability, onto which the superconductor is to be deposited, and also the combination of such bulk material and a so-called buffer layer overlaying the bulk material, if any. In short, in the following, substrate refers to that which supports the superconductor film.
The conventional substrate for high-T.sub.c superconductor thin films consists of a single crystal selected from those materials that normally have a relatively small lattice mismatch with respect to the superconductor material. Traditional materials of this sort are strontium titanate SrTiO.sub.3, magnesium oxide MgO, yttrium-stabilized cubic zirconium oxide ZrO.sub.2, and lanthanum aluminate LaAlO.sub.3, for example. These compounds have a lattice mismatch in the 10.sup.-1 to 10.sup.-3 range. For example, the a-axis (b-axis) lattice parameter of the most widely used compound, YBa.sub.2 Cu.sub.3 O.sub.7--.delta., is 0.382 nm (0.389 nm), whereas the lattice parameters of SrTiO.sub.3, MgO, ZrO.sub.2 and LaAlO.sub.3 substrates are 0.3905 nm, 0.4213 nm, 0.3607 nm, and 0.3793 nm, respectively, at room temperature.
It is known that differences in physical properties between a substrate and a superconductor film borne by the substrate, such as differences in the thermal expansion coefficients, ordinarily give rise to mechanical stress and strain. Reference is made to an article by H. Hidaka et al. entitled "The Stress-Strain Relationship for Multilayers of the High T.sub.c Superconducting Oxides" published in Advances in Superconductivity, Proc. of the 1st International Symposium on Superconductivity (ISS 88), Nagoya 1988, (Springer-Verlag 1989), pp. 581-586. This article points to the importance of a stressless substrate/superconductor relationship, in particular in connection with thin superconductor films.
Similar to differences in physical properties, differences in lattice parameters between a substrate and a superconductor generally lead to stress and/or grain boundary problems in the superconductor film. Several proposals to overcome these problems have already been made. One approach is disclosed in PCT-A-WO 90/04857, where a great many materials that are chemically compatible with the superconductor material have been investigated for closeness of their lattice parameters. Of all materials considered, none was reported to have a lattice constant that perfectly matched the lattice constant of the superconductor material in question.
European patent Appln. No.91 810 336.7 also disclosed the use of nearly-matched materials. Specifically, the application disclosed the use of metallic compounds that had a lattice constant roughly equal to the lattice constant one of the superconductor material to be used. In superconductor/normal-conductor/superconductor ("SNS") heterostructures, such as, e.g. Josephson junctions and field-effect transistors, it was proposed to use, for the metallic substrate, for the insulating layer, if any, and for the high-T.sub.c superconductor layer, materials having at least approximately matching crystal structures and lattice constants. Specifically, the application disclosed that the electrically conductive substrate consist of a metallic oxide such as strontium ruthenate Sr.sub.2 RuO.sub.4, whereas the superconductor layer was disclosed to be of the copper oxide type and may be YBa.sub.2 Cu.sub.3 O.sub.7--.delta., for example. According to the European application, the insulator layer may consist of SrTiO.sub.3.
While the intended approximation of the lattice parameters achieved with the materials suggested by the prior art has led to increased quality of the superconductor thin films, there has heretofore remained a certain degree of mismatch which has impeded the manufacture of more perfect thin films.
It is an object of the present invention to provide a simple method for the manufacturing of substrates that closely approach essentially ideal compatibility with the superconductor materials used as concerns their lattice parameters.